> I noticed that it ran painfully slow on devices with slower disks. I suppose that is going to change now

No, most of the time is spent fsync'ing and it is a problem at a lower layer (related to dpkg), but ot apt. Too many bug reports to list but see [0] for the latest one.

You can use apt with eatmydata as a workaround.

[0] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=613428